Aqueous coating agent, process for its preparation, and its use for coating cans, based on acrylic monomers, epoxy-polyesters and methylol functional curing agents

ABSTRACT

The invention relates to an aqueous coating agent, obtained from an epoxy resin, ethylenically unsaturated monomers, some of which contain carboxyl groups, a peroxide initiator in a proportion of at least 2% by weight, relative to the total weight of the monomers, a crosslinking agent, a neutralizing agent, organic solvents, and, if appropriate, further conventional additives, such as plasticizers, stabilizers, wetting agents, dispersion auxiliaries, catalysts and pigments. The coating agent is based on a binder a) which is obtainable from 
     A) 20 to 80% by weight of an epoxy resin having an average of more than one epoxy group per molecule and having an average molecular weight of at least 500, 
     B) 1 to 60% by weight of polyester polycarboxylic acids having an average molecular weight of 500 to 5,000 and having an acid number of 30 to 150, and 
     C) 10 to 50% by weight of ethylenically unsaturated monomers, 10 to 50% by weight of the monomers containing carboxyl groups, 
     where the sum of A), B) and C) is 100% by weight, the binder a) has an acid number of 20 to 150, and the crosslinking agents b) used are phenolic and/or amino resins, with the proviso that the coating agent contains 
     a) 30 to 70% by weight of the binder a), 
     b) 2 to 30% by weight, preferably 5 to 16% by weight, of the crosslinking agent b), 
     c) 1 to 7% by weight, preferably 2 to 5% by weight, of ammonia and/or amine as neutralizing agent, and 
     d) 20 to 60% by weight of organic solvents, 
     where the sum of a), b), c) and d) is 100% by weight. The present invention also relates to a process for the preparation of the coating agents, and the use thereof for coating of cans.

This application is a division of application Ser. No. 327,964, filedFeb. 14, 1989 now U.S. Pat. No. 4,997,865.

The invention relates to an aqueous coating agent obtained from an epoxyresin, ethylenically unsaturated monomers, some of which containcarboxyl groups, a peroxide initiator, a crosslinking agent, aneutralizing agent, organic solvents, and, if appropriate, furtherconventional additives, such as plasticizers, stabilizers, wettingagents, dispersion auxiliaries, catalysts and pigments.

High-molecular-weight epoxy resins are suitable, in particular, forinternal protective lacquers for tinplate packaging. The crosslinkingagents used are, for example, phenol-formaldehyde, melamine and urearesins. Due to the prespecified application viscosity, such coatingagents based on solvents have a solvent content which is usually between70 and 60%. If--as in the coating of two-part drink cans--it isnecessary to carry out the application of the coating by spraying, afurther increase in the solvent content usually results, which has theconsequence of great pollution through solvent emissions.

In contrast to this, the advantages of aqueous coating systems are to beseen in a markedly reduced solvent emission. In this connection, theapplication of aqueous synthetic resin dispersions by means ofelectrocoating is particularly advantageous, since a virtually 100%coating yield and a further reduced emission of solvents can be achievedusing this method. In addition, it is possible to coat a very widevariety of can geometries using electrophoretic coating through theeffect of the throwing power of electrodeposition coatings, a uniformcoating thickness, and thus also good edge coverage, being achieved, incontrast to coating application by spraying. In addition, theelectrocoating process offers the best prerequisites for processautomation, this process additionally offering the opportunity forsavings besides the reduced material requirement.

As is known, electrocoating can be employed both for anionic and forcationic binder systems. However, in the case of contact withfoodstuffs, for example for internal coatings of cans, it must beremembered that internal protective coatings must meet strict legalrequirements regarding foodstuffs. In addition, such coatings must bestable on storage in contact with the contents, which are mainly acidicto neutral. Taking into account these requirements, anodicelectrocoating is, in principle, more advantageous than the cathodicversion since cathodically deposited films usually contain amine groupsand can thus give rise to stability weaknesses on contact with acidiccontents.

Solvent containing internal protective coatings for cans which are basedon combinations of epoxy resins and phenol-formaldehyde resins or aminoresins and which have good properties have long been known for coatingcans. In particular, epoxy resins which are based on bisphenol A andhave average molecular weights of more than 3,000 g/mol give rise tovery resistant coatings, whereas phenolformaldehyde resins make adecisive contribution to stability against acidic and sulfur-producingcontents.

In order to use such systems in an aqueous medium, the epoxy resin mustbe modified by incorporation of solubilizing groups in a fashion suchthat a water-soluble or water-dispersible system is produced. Cationic,aqueous systems can be obtained in a known fashion by reaction of epoxyresins with amines. For the preparation of anionically dissolvedsynthetic resins, a carboxyl functionality is usually introduced. Tothis purpose, the epoxy resin, as described, for example, in U.S. Pat.No. 3,862,914, is converted into a carboxyl-functional polymer by meansof a reaction with polycarboxylic anhydrides. Such systems, in whichpolycarboxylic acids are bound to polymers via monoester functions, arehowever extremely susceptible to hydrolysis, which causes the storagestability of the corresponding aqueous dispersions of such polymers tobe too low (E. T. Turpin, J. Paint Technol., Vol. 47, No. 602, page 40,1975). Hydrolysis-stable attachment of the carboxyl functionality to theepoxy resin can be achieved according to U.S. Pat. No. 3,960,795 byreacting the epoxy functions with parahydroxybenzoates with formation ofan ether bond, followed by hydrolysis of the benzoate with liberation ofthe carboxyl functionality. The disadvantage of this method is that, inparticular, the high-molecular-weight epoxy resins which are requiredfor internal protective coatings for cans cannot be functionalized withcarboxyl groups by this route to the extent necessary for an aqueousdispersion, as a consequence of their low content of epoxy groups.

U.S. Pat. No. 4,247,439 and European Patents 6334 and 6336 disclosehydrolysis-stable aqueous internal protective coatings for cans, whichcoatings are obtained from products of the esterification of epoxyresins using carboxyl-functional polyacrylate resins. In addition,hydrolysis-stable, aqueous internal protective coatings for cans havebeen disclosed by U.S. Pat. Nos. 4,212,781 and 4,308,185.

The genus-forming U.S. Pat. No. 4,212,781 discloses resin mixtures whichare dispersible in an aqueous, basic medium and which are obtained bycopolymerization of ethylenically unsaturated monomers, some of whichcontain carboxyl groups, in the presence of an aliphatic or aromatic1,2-diepoxy resin using at least 3% by weight, relative to the weight ofthe monomer, of benzyl peroxide or equivalent initiators. The resinmixtures disclosed by U.S. Pat. No. 4,212,781 can be crosslinked usingamino resins. They are suitable, in particular, for spray coating ofdrink cans.

German Offenlegungsschrift 3,446,178 discloses water-dilutablecompositions for coating of metal cans, the polymer present in thecomposition comprising a product of the reaction of acrylic monomers, ahigh-molecular-weight epoxy resin, a phenol-formaldehyde resin and afree-radical initiator.

The aqueous systems known from the prior art are employed mainly forspray coating of two-part aluminum drink cans. They have thedisadvantage that they offer inadequate surface protection on difficultsubstrates, such as, for example, drawn and ironed drink cans made fromtinplate.

The object of the present invention was to provide an aqueous coatingagent for coating metal cans, where universal applicability of thecoating agents is to be guaranteed, i.e. the coating agents must besuitable for coating cans made from aluminum, tinplate and otherspecifically surface-pretreated steel. In particular, the coating oftwo-part drink cans is considered, but, in addition, also the coating offood cans, which need to be stable to a wide range of contents, evenunder sterilization conditions. The new coating systems are alsointended to offer adequate surface protection on difficult substrates.Substrates which are regarded as difficult here are, for example, drawnand ironed tinplate cans which have a thin tin coating and whosesurface, as is known, comprises iron, a little free tin and variousiron-tin alloys. In particular, the aqueous dispersions are intended tobe storage-stable, and they should allow themselves to be readilypigmented. Coating agents prepared therefrom should allow themselves tobe applied without flaws by spray coating and also by anodicelectrocoating. In the case of electrocoating, the binders mustcoagulate at the can, connected as the anode, under the influence of theelectrode reactions to form a closed coating film which has the highestpossible film resistance. In this process, all coating agent components,such as crosslinking agents, auxiliaries and, if appropriate, pigmentsmust be deposited in the amount ratio in which they are also present inthe dispersion. In most systems of the prior art, the problem occursthat the neutral crosslinking agent is not deposited to the extent thatit is present in the aqueous dispersion.

A further requirement of the coating agents to be prepared is that theelectrodeposition coatings should make possible coating times of betweenabout 0.5 and 30 seconds, taking into account the circumstances inindustrial can manufacture. Under these conditions, it must be possibleto produce film thicknesses of between about 4 and 10 μm which aretypical for tinplate packaging. To accomplish this, the wet-filmresistance must be at least 10⁸ Ω⁻¹ cm⁻¹. The throwing power of theelectrodeposition coating should be so well developed that it ispossible to coat even complicated can geometries with an impermeablecoating film of constant coating thickness. Furthermore, the currentstrength/voltage characteristics of the electrocoating materials must bematched to electrode geometries which can be used in practice.

The wet deposited films should be sufficiently hydrophobic to make itpossible to rinse the cans with common rinsing media, such as distilledwater, drinking water, ultrafiltrate, and to exclude redissolution inthe electrocoating material.

The baked coating films should at least reach the property levels ofconventional internal protective coatings for cans with respect tofreedom from pores, stability towards the contents, adhesion to themetal, hardness, elasticity and flavor neutrality, or should surpassthese levels. To this purpose, the residual monomer contents in thebinders must, if appropriate, be kept as small as possible by suitablepreparation processes. For first assessment of the contents stability inthe form of short tests, the pasteurization or sterilization stabilityof baked coating films towards various test solutions--in the simplestcase towards water--is important here.

The object of the present invention is achieved by the aqueous coatingagent of the type mentioned initially, wherein the coating agent isbased on a binder a) which is obtainable from

A) 20 to 80% by weight of an epoxy resin having an average of more thanone epoxy group per molecule and having an average molecular weight ofat least 500,

B) 1 to 60% by weight of polyester polycarboxylic acids having anaverage molecular weight of 500 to 5,000 and having an acid number of 30to 150, and

C) 10 to 50% by weight of ethylenically unsaturated monomers, 10 to 50%by weight of the monomers containing carboxyl groups,

where the sum of A), B) and C) is 100% by weight, the peroxide initiatoris employed in a proportion of at least 2% by weight, relative to thetotal weight of the monomers C), the binder a) has an acid number of 20to 150, and the crosslinking agents b) used are phenolic and/or aminoresins, with the proviso that the coating agent contains

a) 30 to 70% by weight of the binder a),

b) 2 to 30% by weight, preferably 5 to 16% by weight, of the phenolicand/or amino resin b),

c) 1 to 70% by weight, preferably 2 to 5% by weight, of ammonia and/oramine as neutralizing agent, and

d) 20 to 60% by weight of organic solvents,

where the sum of a), b), c) and d) is 100% by weight.

As component A), polyglycidyl ethers of bisphenol A having an averagemolecular weight of 500 to 20,000 are preferably employed. Examples ofsuitable epoxy resins are glycidyl polyethers, which are marketed, forexample, under the tradenames Epikote 1001, 1004, 1007 and 1009. Theepoxy resins (component A) advantageously have an average molecularweight of at least 3,000 g/mol.

The polyester polycarboxylic acids employed as component B) are preparedunder the conditions known to those skilled in the art forpolyesterification reactions. These compounds are known polycondensatesmade from aromatic and/or aliphatic dicarboxylic acids, aromaticdicarboxylic anhydrides, aromatic tricarboxylic anhydrides, aromatictetracarboxylic anhydrides and dianhydrides, and aliphatic andcycloaliphatic mono-, di- and triols. Preferred starting compounds forthe polyester polycarboxylic acids are terephthalic acid, isophthalicacid, trimellitic acid, trimellitic anhydride, adipic acid, sebacicacid, aliphatic monools having 4 to 20 carbon atoms,2,2-dimethyl-1,3-propanediol, ethylene glycol, diethylene glycol,trimethylol propane, glycerol and pentaerythritol.

The polyester polycarboxylic acids B) preferably have an averagemolecular weight of 1,000 to 3,000 and an acid number of 50 to 100.

A preferred embodiment of the polyester polycarboxylic acid component B)comprises using ester diols and/or glycidyl esters of monocarboxylicacids as the polyol component for the preparation of the polyesterpolycarboxylic acids. Neopentyl glycol hydroxypivalate may be mentionedas an example of a suitable ester diol. A suitable commerciallyavailable glycidyl ester of monocarboxylic acids is the glycidyl esterof versatic acid, a branched monocarboxylic acid.

The polyester polycarboxylic acids prepared using ester diols and/orglycidyl esters of monocarboxylic acids have acid numbers in the rangefrom 100 to 130.

10 to 50% by weight of the ethylenically unsaturated monomers employedas component C) are monomers containing carboxyl groups. Examples whichshould be mentioned of monomers containing carboxyl groups are acrylicacid and methacrylic acid. In addition, nonfunctionalized monomers, suchas, for example, styrene, vinyltoluene and α-methylstyrene, may beemployed as monomers.

(Meth)acrylates having 1 to 20 carbon atoms in the alcohol radical arepreferably used as the third class of monomers, it also being possibleto employ hydroxy-functional monomers. Examples of these are ethylacrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutylacrylate, t-butyl acrylate, pentyl acrylate, decyl acrylate, laurylacrylate, methyl methacrylate, butyl methacrylate, isobutylmethacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, octylmethacrylate, nonyl methacrylate, hydroxyethyl acrylate, hydroxypropylacrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate and hydroxybutyl methacrylate.

The ethylenically unsaturated monomers of component C) preferablycomprise

x) 10 to 50% by weight, preferably 20 to 40% by weight, of monomerscontaining carboxyl groups,

y) 0 to 50% by weight, preferably 20 to 40% by weight, ofnonfunctionalized monomers, and

z) 5 to 60% by weight, preferably 10 to 50% by weight, of(meth)acrylates, having 1 to 20 carbon atoms in the alcohol radical,and, if appropriate, being hydroxy-functional,

where the sum of x), y) and z) is 100% by weight.

Component C) has an acid number in the range from 30 to 150, preferablyin the range from 50 to 100.

The binder a) is preferably obtained from 35 to 60% by weight of A), 10to 35% by weight of B) and 15 to 30% by weight of C), where the sum ofA), B) and C) is 100% by weight.

It is preferred that at least 2.6% by weight, particularly preferably atleast 3% by weight, relative to the total weight of the ethylenicallyunsaturated monomers, of peroxide initiators are employed.

According to the present invention, any phenolic resin can be used solong as it has the methylol functionality which is necessary for thereactivity. Preferred phenolic resins are products, prepared underalkaline conditions, of the reaction of phenol, substituted phenols andbisphenol A with formaldehyde. Under such conditions, the methylol groupis linked to the aromatic ring either in the ortho or in the paraposition.

Phenolic resins of the resol type which are based on bisphenol A andcontain more than one methylol group per phenyl ring are preferablyemployed.

Typical amino resins are melamine, benzoguanamine and urea-formaldehyderesins. These are preferably used in the form which has been etherifiedwith lower alcohols, usually butanol. Suitable amino resins arecommercially available, for example, under the tradename Cymel. Asuitable amino resin is, for example, hexamethoxymethylmelamine.

Of course, besides condensation products with formaldehyde, those withother aldehydes can also be used.

According to the invention, the coating agent contains 1 to 7% byweight, preferably 2 to 5% by weight, of ammonia and/or amines asneutralizing agents. The coating agent becomes dispersible in waterthrough neutralization with component c). Triethylamine and/ordimethylethanolamine are preferably employed as neutralizing agents.

The aqueous coating agents according to the invention furthermorecontain 20 to 60% by weight of organic solvents. When the aqueouscoating agents are used as anodic electrodeposition coatings, it must beensured that the organic solvents positively influence the effectivenessof the anodic deposition and the flow of the coating film. In apreferred fashion, nonvolatile cosolvents are employed, such asmonoalcohols having 4 to 18 carbon atoms, glycol ethers, such as, forexample, ethylene glycol monoethyl ether and its higher homologs having5 to 20 carbon atoms, or corresponding ethers of 1,2-and1,3-propanediol.

The coating agents according to the invention and described above areprepared in a process wherein the epoxy resin A) is initially reactedwith the polyester polycarboxylic acid component B) at 80° to 200° C.,preferably at 120° to 180° C., with the use of catalysts, so that atleast 80% of the oxirane rings initially present are opened, componentC) is subsequently subjected to free-radical polymerization, in thepresence of the reaction product obtained in the first process step, at100° to 160° C., preferably 120° to 140° C., with the use of at least 2%by weight, relative to the weight of the ethylenically unsaturatedmonomers, of peroxidic initiators preferably ones which generatebenzoyloxy and/or phenyl free radicals, the product obtained isneutralized in a third process step using component c), and the organicsolvent d), the crosslinking agent b) and, if appropriate, furtherconventional additives are added and mixed, and the coating agent isdispersed in water.

The reaction of the epoxy resin with polyester polycarboxylic acidstaking place in the first process step is catalyzed by amines,preferably tertiary amines. The reaction is carried out in a fashionsuch that at least 80% of the oxirane rings are converted intoβ-hydroxyester groups.

In the second process step, the ethylenically unsaturated monomers, someof which contain carboxyl groups, of component C) are subjected to afree-radical polymerization reaction in the presence of theβ-hydroxyester produced in the first process step. The free-radicalpolymerization is initiated by at least 2% by weight, relative to thetotal weight of the monomers, of peroxidic initiators, preferably oneswhich generate benzoyloxy and/or phenyl free radicals. In this reaction,at least 2.6% by weight of initiators are preferably used, particularlypreferably at least 3% by weight. Of course, good results are alsoachieved when high proportions, for example 8 to 10% by weight, ofinitiators are employed, but this is not recommended for economicreasons. If the polymerization is carried out in the presence ofrelatively low initiator concentrations, for example with less than 3%by weight, relative to the monomer weight, a higher degree ofneutralization is necessary in order to obtain a stable dispersion (cf.Example 3 from Table 1).

Primarily, peroxidic initiators are employed which decompose to producebenzoyloxy and/or phenyl free radicals. Of course, it is also possibleto use other initiators so long as these lead to equivalent free-radicalconditions.

Preferred initiators are dibenzoyl peroxide and/or tert. butylperbenzoate. Further possible initiators which should be mentioned aretert. butyl peroctoate, cumene hydroperoxide and methyl ethyl ketoneperoxide.

The proportion of residual monomers is advantageously kept to less than0.2%, relative to the sum of a) to d), by adding further initiatorand/or by extending the initiator feed time.

After the free-radical polymerization, the polymer obtained isneutralized in a third process step in order to make the coating agentwater-dispersible. The nonvolatile cosolvent d) necessary for productionof a readily flowing, anodically deposited film, the phenolic resins oramino resins b) used as crosslinking agents, and further additives whichare conventional, for example, in electrocoating are added and mixedwith the system. Finally, the system is dispersed in water.

A preferred embodiment of the process according to the inventioncomprises carrying out a precondensation with the crosslinking agent b)after the free-radical polymerization. In this fashion, the crosslinkingagent b) is also deposited during electrocoating to the same extent asit is present in the aqueous coating agent.

The mixture obtained before dispersal in water can be used as acompensation coating by not preparing the aqueous dispersion until thebinder is incorporated into the electrodeposition coating.

A preferred embodiment of the process according to the inventioncomprises already using the organic solvent d) as a solvent in theesterification, occurring as the first process step, of the epoxy resinA) and the polyester polycarboxylic acids B).

The aqueous coating agents according to the invention are advantageouslyused for anodic electrocoating of cans and can halves. Of course, theycan also be employed for spray coating of cans. In anodicelectrocoating, the cans are dipped in an aqueous bath based on thecoating agent according to the invention described above and areconnected as the anode.

By means of direct current, a film is deposited on the cans, thesubstrate is removed from the bath, and the film is hardened by baking.

Both in spray coating and in electrodeposition, the final hardening ofthe coating film is carried out by baking.

The aqueous coating agents according to the invention are suitable forcoating of cans which can comprise different materials and which canhave a very wide variety of can geometries. Thus, cans made fromaluminum and those made from tinplate, for example drawn and ironed,two-part drink cans, can be coated equally well using the coating agentsaccording to the invention. In addition, cans made fromsurface-pretreated steel sheeting can be coated excellently.

The aqueous coating agents described above are likewise highly suitablefor coating foodstuff cans which have been drawn and ironed ordeep-drawn in another fashion and which are subjected to sterilizationfor preservation of the contents.

The can halves discussed are bodies and lids which are used for themanufacture of foodstuff cans. Anodic coating of the can halves hasproven particularly advantageous when the bodies are welded and the lidsare pull-tab lids.

The advantages of the process according to the invention are that thereis a wide variety of possible ways of controlling the acid number byvarying the polyester or the polymer. In this fashion, applicationalproperties and adhesion properties can be optimized for specific metalsurfaces. The compatibility of the components with one another and thesafety with respect to residual monomers are ensured by thepolymerization process.

The aqueous coating agents according to the invention are storage-stableand can be applied without flaws by means of anodic electrocoating. Thebaked coating films obtained have a good property level with respect tofreedom from pores, stability towards the contents, adhesion to themetal, hardness, elasticity and flavor neutrality. In addition, thebinder combinations employed enable good pigment wetting.

The invention is described in greater detail below with reference toillustrative embodiments:

1. Preparation of a polyester polycarboxylic acid

1.1 1,330 g of isophthalic acid, 145 g of adipic acid, 780 g of2,2-dimethyl-1,3-propanediol, 268 g of trimethylolpropane and 200 g ofisodecanol are weighed out into a four-neck flask fitted with stirrer,thermometer and water separator, and the mixture is condensed at 220° C.to an acid number of less than 5 mg of KOH/g. 500 g of trimelliticanhydride are added at 170° C., and the batch is kept at thistemperature until the viscosity becomes constant. Finally, the polyesterresin melt is dissolved in butyl glycol to give a 70% strength solution.The acid number is 85 mg of KOH/g.

1.2 1,200 g of the glycidyl ester of versatic acid, 900 g of 2-butanone,900 g of trimellitic anhydride and 5 g of N,N-dimethylbenzylamine arewarmed to 90° C. in a four-neck flask fitted with stirrer, thermometerand reflux condenser. When the viscosity (measured at 23° C.) hasincreased to 1.5 Pas, the batch is cooled and discharged.

2. Preparation of Epoxy Ester Resins

2.1 Preparation of an epoxy ester resin based on the polyesterpolycarboxylic acid prepared under 1.1.

A mixture of 1,050 g of an epoxy resin based on bisphenol A and havingan epoxy equivalent weight of 3,400, 700 g of butyl glycol, 350 g of1-phenoxy-2-propanol, 2 g of N,N-dimethylbenzylamine and 1,000 g of thepolyester polycarboxylic acid prepared under 1.1 is warmed to 160° C. ina four-neck flask fitted with stirrer, thermometer and reflux condenseruntil the acid number has fallen to 20 mg of KOH/g. In a 30% strengthsolution in butyl glycol, the epoxy ester thus prepared has a viscosityof 370 mPa.s at 23° C.

2.2 Preparation of an epoxy ester based on the polyester carboxylic acidprepared under 1.2.

A solution of 1,050 g of an epoxy resin based on bisphenol A and havingan epoxy equivalent weight of 3,400, in 1,000 g of butyl glycol and 440g of propylene glycol monophenyl ether is heated to 140° C. in afour-neck flask fitted with stirrer, thermometer and distillationattachment. After 2 g of N,N-dimethylbenzylamine are added, 950 g of thepolyester polycarboxylic acid prepared under 1.2 are run in and thesolvent (2-butanone) is simultaneously distilled off. The batch is keptat 160° C. for a further 3 hours. The acid number is then 37 mg of KOH/gand the viscosity (of a 30% strength solution in butyl glycol at 23° C.)is 380 mPa.s.

3. Preparation of Binder Solutions from the Epoxy Ester Resins PreparedUnder 2.

3.1 Preparation using the epoxy ester resin prepared under 2.1

EXAMPLE 1

2,400 g of the epoxy ester prepared under 2.1 are placed in a four-neckflask fitted with stirrer, thermometer, reflux condenser and two supplycontainers. At 140° C., a mixture of 130 g of acrylic acid, 160 g ofstyrene and 200 g of butyl acrylate is added to this from the firstsupply container and a solution of 30 g of tert. butyl perbenzoate in 40g of butyl glycol is simultaneously added from the second supplycontainer. The monomers are added over 2 hours and the initiator over 3hours. When the polymerization is complete, 190 g of a highlymethylolated bisphenol A-formaldehyde resin are precondensed with thebatch at 90° C. for 2 hours.

A 58% strength binder solution is produced which, after addition ofbasic neutralizing agents, can be employed directly as a compensationcoating for anodic electrocoating.

EXAMPLE 2

2,400 g of the epoxy ester prepared under 2.1 are placed in a four-neckflask fitted with stirrer, thermometer, reflux condenser and two supplycontainers. At 140° C., a mixture of 130 g of acrylic acid, 160 g ofstyrene and 200 g of butyl acrylate is added to this from the firstsupply container and a solution of 30 g of tert. butyl perbenzoate in 40g of butyl glycol is added simultaneously from the second supplycontainer. The monomers are added over 2 hours, and the initiator over 3hours. When the polymerization is complete, 190 g of a butylatedmelamine-formaldehyde resin are added.

A 58% strength binder solution is produced which, after addition of abasic neutralizing agent, can be employed directly as a compensationcoating for anodic electrocoating.

EXAMPLE 3

2,352 g of the epoxy ester prepared under 2.1 are placed in a four-neckflask fitted with stirrer, thermometer, reflux condenser and two supplycontainers. At 140° C., a mixture of 130 g of acrylic acid, 160 g ofstyrene and 190 g of butyl acrylate is added to this from the firstsupply container and a solution of 13.4 g of tert. butyl perbenzoate in40 g of butyl glycol is added simultaneously from the second supplycontainer. The monomers are added over 2 hours, and the initiator over 3hours. When the polymerization is complete, 190 g of a highlymethylolated bisphenol A-formaldehyde resin are precondensed with thebatch at 90° C. for 2 hours.

A 57% strength binder solution is produced which, after addition ofbasic neutralizing agents, can be employed directly as a compensationcoating for anodic electrocoating.

3.2 Preparation using the epoxy ester resin prepared under 2.2

EXAMPLE 4

2,100 g of the epoxy ester prepared under 2.2 and 300 g of butyl glycolare placed in a four-neck flask fitted with stirrer, thermometer, refluxcondenser and two supply containers. At 140° C., a mixture of 130 g ofacrylic acid, 160 g of styrene and 200 g of butyl acrylate is added tothis from the first supply container and a solution of 30 g of tert.butyl perbenzoate in 40 g of butyl glycol is added simultaneously fromthe second supply container. The monomers are added over 2 hours, andthe initiator over 3 hours. When the polymerization is complete, 190 gof a highly methylolated bisphenol A-formaldehyde resin are precondensedwith the batch at 90° C. for 2 hours.

A 56% strength binder solution is produced which, after addition ofbasic neutralizing agents, can be employed directly as a compensationcoating for anodic electrocoating.

3.3 Comparison Examples

COMPARISON EXAMPLE 1 Modification of an Epoxy Resin Using TrimelliticAnhydride

In order to prepare a comparison batch without addition polymer, thehigh-molecular-weight epoxy resin employed under 2. is reacted withtrimellitic anhydride after esterification of the glycidyl radicalsusing a monocarboxylic acid.

To this purpose, 41.8 parts of a high-molecular-weight epoxy resin basedon bisphenol A and having an epoxy equivalent weight of 3,400 aredissolved in 41.0 parts of ethylene glycol monobutyl ether and reactedat 130° C. with 1.94 parts of isononanoic acid and 0.06 parts ofN,N-dimethylbenzylamine until the acid number has fallen to below 3 mgof KOH/g. 7.7 parts of trimellitic anhydride are added, and thetemperature is maintained until an acid number of 80 mg of KOH/g isreached. After cooling to 90° C., 3.5 parts of a phenol-formaldehyderesin (resol type based on bisphenol A) are added, and the batch isstirred at 90° C. for 2 hours. The solids content is then 55%.

COMPARISON EXAMPLE 2 Preparation of an Acrylic-Epoxy Graft Polymer

In order to prepare a comparison batch, a monomer mixture is polymerizedin the presence of a high-molecular-weight epoxy resin, but in theabsence of a polyester component.

To this purpose, 1,120 g of a high-molecular-weight epoxy resin based onbisphenol A and having an epoxy equivalent weight of 3,400 are dissolvedin 570 g of butyl glycol and 850 g of n-butanol and reacted at 140° C.with 44 g of dimethylolpropanoic acid and 1.5 g ofN,N-dimethylbenzylamine until the acid number has fallen to below 3 mgof KOH/g. A mixture of 175 g of methacrylic acid, 130 g of styrene, 5 gof 2-ethylhexyl methacrylate and 28 g of benzoyl peroxide (75% strength)is added to this at 120° C. within 2 hours. When the polymerization iscomplete, 160 g of a highly methylolated bisphenol A-formaldehyde resinare precondensed with the batch at 90° C. for 2 hours. A 50% strengthbinder solution having a viscosity (30% strength in solution in butylglycol) of 0.8 Pa.s and an acid number of 90 mg of KOH/g is produced.

4. Preparation of Binder Dispersions from the Binders of Examples 1, 2,3 and 4 and Comparison Examples 1 and 2

The binder solutions of Examples 1, 2, 3 and 4 and Comparison Examples 1and 2 are neutralized with amine according to the figures in Table 1,dispersed slowly in demineralized water with vigorous stirring andadjusted to a solids content of 12%. The properties and characteristicnumbers of the resulting dispersions are collated in Table 1.

Binder dispersion E is not adequately stable on storage at roomtemperature. After one month, the binder has substantially coagulatedand the dispersion is destroyed. In contrast, the dispersions A, B, C, Dand F are free of sediment even after storage for 6 months at roomtemperature.

                                      TABLE 1                                     __________________________________________________________________________                                  E      F                                        Dispersion                                                                          A     B     C     D     Comparison                                                                           Comparison                               Binder                                                                              Example 1                                                                           Example 2                                                                           Example 3                                                                           Example 4                                                                           Example 1                                                                            Example 2                                __________________________________________________________________________    Amine N,N-dime-                                                                           N,N-dime-                                                                           N,N-dime-                                                                           N,N-dime-                                                                           Triethyl-                                                                            Triethyl-                                      thyletha-                                                                           thyletha-                                                                           thyletha-                                                                           thyletha-                                                                           amine  amine                                          nolamine                                                                            nolamine                                                                            nolamine                                                                            nolamine                                              Degree of                                                                           80%   80%   98%   80%   53%    49%                                      neutrali-                                                                     zation                                                                        Storage                                                                             >6 mon.                                                                             >6 mon.                                                                             >6 mon.                                                                             >6 mon.                                                                             <1 mon.                                                                              >6 mon.                                  stability                                                                     of the                                                                        dispersion                                                                    (20° C.)                                                               pH    8.2   8.2   8.3   7.8   8.5    7.2                                      Conductiv-                                                                          2000  2200  2540  2700  2000   1750                                     ity (μS/cm)                                                                __________________________________________________________________________

5. Coating of Drink Cans with Binder Dispersions A, B, C, D, E and Ffrom Table 1

5.1 Coating of a drink can with binder dispersion A

EXAMPLE 5

An uncoated, two-part drink can made from tinplate is held at the flangeusing an electroconductive clip, filled with binder dispersion A andsubmerged in a conductive vessel which has a diameter of 20 cm, isinsulated against earth and has previously likewise been filled with theelectrodeposition coating. The positive pole of a direct current voltagesource is connected to the can and the negative pole is connected to theexternal vessel. The coating is carried out using an auxiliary cathodein the can interior. After rinsing with demineralized water, the coatingis baked for 5 minutes at 210° C. in a circulation oven. The can isfully coated internally and externally with a thin, clear, impermeablecoating film. Measurement results, cf. Table 2.

5.2 Coating of a drink can with binder dispersion B

EXAMPLE 6

The coating is carried out analogously to the procedure under 5.1. Thecan is fully coated internally and externally with a thin, impermeable,clear coating film. Measurement results, cf. Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                           Comparison                                                                           Comparison                               Example 5                                                                           Example 6                                                                           Example 7                                                                           Example 8                                                                           Example 9                                                                           Example 3                                                                            Example 4                       __________________________________________________________________________    Solids content                                                                         12%   12%   12.5% 10%   12%   12%    12%                             pH       8.2   7.8   8.2   8.3   7.8   8.5    7.2                             Conductivity                                                                           2000  2700  2200  2540  2700  2000   1750                                                                          μS/cm                        Bath tempera-                                                                          27° C.                                                                       27° C.                                                                       27° C.                                                                       27° C.                                                                       27° C.                                                                       27° C.                                                                        27° C.                   ture                                                                          Deposition                                                                             20 s  20 s  20 s  20 s  20 s  20 s   20 s                            time                                                                          Deposition                                                                             50 V  60 V  60 V  50 V  60 V  160 V  90 V                            voltage                                                                       Coating/can                                                                            480 mg                                                                              490 mg                                                                              900 mg                                                                              500 mg                                                                              490 mg                                                                              450 mg 460 mg                          Surface  glossy                                                                              glossy                                                                              glossy                                                                              glossy                                                                              glossy                                                                              matt   matt                            Porosity, mA                                                                           0.1   0.1   0.3   1     0.1   10     5                               (enamel rater)                                                                Coating adhesion                                                                       Gt 0  Gt 0  Gt 0  Gt 0  Gt 0  Gt 1   Gt 1                            (cross-hatch                                                                  test)                                                                         Sterilization                                                                          OK    OK    OK    OK    OK    water  water                           with water,                            absorption                                                                           absorption                      30 min/121° C.                  (blushing)                                                                           (blushing)                      __________________________________________________________________________

5.3 Coating a drink can with pigmented binder dispersion B

EXAMPLE 7

The binder from Example 2 is pigmented with titanium dioxide(binder:pigment=1:1) and adjusted to a solids content of 12.5% usingdemineralized water. The coating is carried out analogously to 5.1 and5.2. The can is coated completely with a white coating film. Measurementresults, cf. Table 2.

5.4 Coating of a drink can with binder dispersion C

EXAMPLE 8

The coating is carried out analogously to 5.1 and 5.2. The can is fullycoated internally and externally with a thin, impermeable, clear coatingfilm. Measurement results, cf. Table 2.

5.5 Coating of a drink can with binder dispersion D

EXAMPLE 9

Coating is carried out analogously to 5.1 and 5.2. The can is fullycoated internally and externally with a thin, impermeable, clear coatingfilm. Measurement results, cf. Table 2.

5.6 Coating of a drink can with binder dispersion E

COMPARISON EXAMPLE 3

The coating is carried out analogously to Example 4-9. The can is coatedinternally and externally with a clear, matt coating film which is notimpermeable and has surface defects. Measurement results, cf. Table 2.

5.7 Coating of a drink can with binder dispersion F

COMPARISON EXAMPLE 4

The coating is carried out as described above. The can is coatedinternally and externally with a clear, matt coating film which is notimpermeable and which has surface defects. Measurement results, cf.Table 2.

The deposited and baked coating films in all examples exhibit no odor,flavor or color impairment of water as the contents. Similar results areachieved when a drink can made from aluminum is used in place of atwo-part drink can made from tinplate.

6. Coating of a drink can by means of spray coating using binderdispersion A

The interior of a two-part drink can made from tin-plate is spray-coatedwith anionic binder dispersion A. 65 bar is selected as the sprayingpressure. The coating is baked for 2 minutes at 210° C. in a circulationoven. An applied coating layer of 220 mg dry/0.33 l can is produced. Thecoating film is clear and glossy and has a porosity (enamel rater) of0.8 mA. The other coating properties (adhesion and sterilizationstability) correspond to those of Examples 5 to 9 from Table 2.

We claim:
 1. A process for the preparation of an aqueous coating agentwherein the coating agent comprises:(a) 30 to 70% by weight of a binderhaving an acid number of 20 to 150 and comprised of the reaction productof:(A) 20 to 80% by weight of an epoxy resin having an average of morethan one epoxy group per molecule and having an average molecular weightof at least 500, (B) 1 to 60% by weight of a polyester polycarboxylicacid having an average molecular weight of 500 to 5,000 and having anacid number of 30 to 150, and (C) 10 to 50% by weight of ethylenicallyunsaturated monomers, 10 to 50% of which contain carboxy groups, and aperoxide initiator which is present in an amount which is at least 2% byweight relative to the weight of the ethylenically unsaturated monomers,the sum of (A), (B), and (C) being 100% weight; (b) 2 to 30% by weightof a phenolic or amino resin crosslinking agent; (c) 1 to 7% by weightof ammonia or an amine neutralizing agent, and (d) 20 to 60% by weightof an organic solvent, the sum of a), b), c) and d) being 100% byweight;the process comprising the steps of:
 1. reacting the epoxy resinA) with the polyester polycarboxylic acid component B) at a temperatureof about 80° to 200° C. in the presence of catalyst so that at least 80%of the oxirane rings initially present are opened;2.
 2. polymerizingcomponent C) in the presence of the reaction product of step 1 at atemperature of about 100° to 160° C.;3. neutralizing the polymerizationproduct of step 2 using component (c);
 4. adding the organic solvent d)and the crosslinking agent b) to the neutralized product of step 3 toform the coating agent, and
 5. dispersing the coating agent in water. 2.A process as claimed in claim 1, wherein the peroxide initiator is atleast 2.6% by weight relative to the total weight of the ethylenicallyunsaturated monomers.
 3. A process as claimed in claim 1, wherein theperoxide initiator is at least 3.0% by weight relative to the totalweight of the ethylenically unsaturated monomers.
 4. A process asclaimed in claim 1, wherein the epoxy resin A) is based on bisphenol A.5. A process as claimed in claim 1 wherein the epoxy resin A) has anaverage molecular weight of at least 3,000.
 6. A process as claimed inclaim 1, wherein the polyester polycarboxylic acid B) has an averagemolecular weight of 1,000 to 3,000 and an acid number of 50 to
 100. 7. Aprocess as claimed in claim 1, wherein the polyester polycarboxylic acidcontains an alcohol component which is an ester diol and/or a glycidylester of a monocarboxylic acid.
 8. A process as claimed in claim 1,wherein the ethylenically unsaturated monomers C) comprisex) 10 to 50%by weight of a monomer containing carboxyl groups, y) 0 to 50% by weightof a nonfunctionalized monomer, and z) 5 to 60% by weight of a(meth)acrylate having 1 to 20 carbon atoms in the alcohol radical wherethe sum of x), y) and z) is 100% by weight.
 9. A process as claimed inclaim 1, wherein the ethylenically unsaturated monomers C) comprisex) 20to 40% by weight of a monomer containing carboxyl groups, y) 20 to 40%by weight of a nonfunctionalized monomer, and z) 10 to 50% by weight ofa (meth)acrylate having 1 to 20 carbon atoms in the alcohol radicalwhere the sum of x), y) and z) is 100% by weight.
 10. A process asclaimed in claim 1 wherein the binder a) is comprised of from 35 to 60%by weight of A), 10 to 35% by weight of B) and 15 to 30% by weight ofC).
 11. A process as claimed in claim 1 wherein b) is a phenolic resinresol based on bisphenol A which contains more than one methylol groupper phenyl ring.
 12. A process as claimed in claim 1 wherein theneutralizing agent c) is triethylamine or dimethylethanolamine.
 13. Theprocess as claimed in claim 1, wherein the polymerization product fromstep 2 is precondensed with the crosslinking agent b) prior to theneutralization step
 3. 14. The process as claimed in claim 1, whereinless than 0.2% by weight, relative to the sum of components a) to d), ofresidual monomers are present after the polymerization step
 2. 15. Theprocess as claimed in claim 1, wherein the peroxide initiator isdibenzoyl peroxide and/or tert. butyl perbenzoate.
 16. The process asclaimed in claim 1, wherein component d) is used a solvent in theesterification process of step
 1. 17. The process of claim 1, whereinthe reaction of epoxy resin A) and the polyester polycarboxylic acid (B)in step 1 is at a temperature of about 120° to 180° C.
 18. The processof claim 1, wherein the polymerization of component C) in step 2 is at atemperature of about 120° to 140° C.
 19. The process of claim 1, whereinthe peroxidic initiator generates benzoyloxy or phenyl free radicals.20. The process of claim 1, wherein the coating agent comprises 5 to 16%by weight of component (b).
 21. The process of claim 1, wherein thecoating agent comprises 2 to 5% by weight of component (c).